How Does PCB Printing Work?
PCB printing, specifically the method involving photo films, is a crucial step in transferring the electronic design onto the physical board. It involves creating highly accurate representations of the circuit layers that will guide the manufacturing process.
The printing process for creating the circuit pattern on a PCB doesn't involve directly printing ink onto the board like a paper printer. Instead, it focuses on creating tools—high-precision films—that are then used in a photographic process to transfer the design onto the copper layers of the PCB material.
Here's a breakdown based on common manufacturing practices and the provided reference:
Starting the Journey: Design to Manufacturing
The process begins after the design phase is complete. PCB designers output the necessary digital files representing the circuit layers, drill patterns, silkscreen labels, and solder mask. These files are then sent to the manufacturer. A vital preliminary step is the Design for Manufacturing (DFM) check. This ensures the design is feasible and optimized for the manufacturer's specific processes and equipment.
Creating the Photo Films
Once the DFM check is approved, the printing stage commences. Manufacturers utilize a specialized piece of equipment known as a plotter.
- What is a plotter? In the context of PCB manufacturing, a plotter is a high-resolution printer designed to create extremely accurate negatives or positives of the PCB layers on transparent film sheets.
- The Output: Manufacturers use a special printer called a plotter, which makes photo films of the PCBs, to print circuit boards. Each layer of the PCB design (copper traces for each side, solder mask, silkscreen) typically requires its own separate film. These films act like stencils for light exposure in later steps.
Using Films for Imaging
With the precise photo films created by the plotter, the manufacturing process moves to the imaging phase. This is where the circuit pattern is transferred onto the copper-clad laminate material that forms the base of the PCB.
- How Films are Used: Manufacturers will use the films to image the PCBs. This typically involves placing the film over the copper-clad board which has been coated with a photosensitive material (photoresist).
- The Exposure: The board with the film is then exposed to UV light. The light passes through the transparent areas of the film but is blocked by the opaque areas (which represent the circuit pattern or the areas to be protected). This exposure hardens or softens the photoresist in the exposed areas, depending on the type of photoresist used.
- Development: After exposure, the board is put through a developer solution. This solution removes either the exposed or unexposed photoresist, leaving behind the desired pattern of photoresist on the copper surface.
- Etching: The board then goes through an etching process, typically using a chemical solution. This etchant removes the copper that is not protected by the remaining photoresist. The areas protected by the photoresist are left intact, forming the circuit traces and pads.
- Photoresist Removal: Finally, the remaining photoresist is stripped away, revealing the copper circuit pattern underneath.
In summary, the "printing" in this method refers to the high-precision plotting of films, which are then used in a photographic imaging process to transfer the design onto the copper, ultimately defining the conductive pathways of the circuit board.
Steps in Film-Based Imaging:
- Design & DFM: Prepare the digital design files.
- Film Plotting: Use a plotter to create accurate photo films for each layer.
- Imaging: Place films over photoresist-coated board and expose to UV light.
- Develop: Remove photoresist from either exposed or unexposed areas.
- Etch: Remove unprotected copper.
- Strip: Remove remaining photoresist.
This film-based method, while still used, is being complemented or replaced by more modern techniques like Direct Imaging (DI), which uses lasers to write the pattern directly onto the photoresist without needing physical films. However, the fundamental concept of transferring a pattern onto the copper layer remains central to PCB manufacturing.
